The present invention relates to a piston engine powertrain, and more particularly to a powertrain which transmits power from a piston of an internal combustion engine to an output shaft. This invention is an improvement over the powertrain disclosed in pending U.S. patent application Ser. No. 09/377,863, filed Aug. 20, 1999.
Modern mid-size sedans can maintain a highway speed of 70 mph with only one-third of the installed engine power. Full power is required only for quick acceleration and climbing a steep slope. Therefore, it is desirable to provide a powertrain which permits one or more cylinders to be deactivated when power is not needed and which enables the deactivated cylinders to be reactivated to restore power quickly when power is required. On average, each cylinder will then operate only one-third of time, and therefore engine life is increased three fold.
For a given maximum pressure, a constant-pressure cycle compression-ignition (CI) engine has a higher efficiency, a greater specific power, and a lower firing temperature than either a constant-volume or limited-pressure cycle engine. The firing temperature at the end of a constant pressure combustion process can be limited by selecting an appropriate compression ratio instead of recycling the exhaust gas (EGR). The lost cycle efficiency due to limited firing temperature can be recovered with a much larger expansion ratio than compression ratio. Therefore, it is desirable to have a powertrain which enables the coordination of piston movement with the rate of fuel injection to obtain constant pressure combustion. At the same time, the powertrain can provide two different piston strokes, a short stroke for intake and compression processes and a long stroke for expansion and exhaust processes.
In the case of a conventional piston-crank assembly powertrain in which the piston is permanently connected to a crankshaft, only stopping the engine can stop piston motion. The same crank radius determines the length of both the compression and expansion strokes, and therefore the length of the two strokes cannot differ. The traditional piston-crank assembly powertrain should accordingly be replaced by an alternative powertrain. Of the available alternative powertrain, the piston-cam assembly powertrain offers the greatest potential for achieving the desired results. As early as 1927, The Fairchild-Caminez 4-cylinder engine with a piston-cam assembly powertrain was built and successful endurance tested. However, such a prior art powertrain needs modification for present purposes.
The present invention is an advanced piston-cam assembly powertrain similar to that shown in U.S. patent application Ser. No. 09/377,863, now Pat. No. 6,125,802, but with certain elements modified to provide unique improved results. The present invention incorporates two separate parts, namely a reciprocating part and a rotary part. The reciprocating part includes a piston, connecting rod and lever. The lever of the invention is formed of a strong elastic material such as spring steel for a purpose hereinafter described. The lower end of the connecting rod is pivotally connected to the free end of a lever to define a pivot axis. A drive means in the form of two spaced rollers is mounted on the free end of the lever for rotation about the aforementioned pivot axis. The other end of the lever is pivotally supported on the engine body. The rotary part of the invention includes a member which is drivingly connected to a rotatable output shaft and has a cam surface formed thereon. The cam surface defines a profile having two lobes with different base circles which produces two different piston strokes comprising a short stroke for intake and compression processes and a long stroke for expansion and exhaust processes.
The pivotally supported end of the lever can be locked in two different operative positions. In one operative position, the lever is straight and the drive rollers are disposed out of contact with the cam surface so that the associated piston does not reciprocate within its cylinder in the engine body, and accordingly the cylinder is deactivated. In the other operative position, the lever is bent and the drive rollers are disposed in continuous contact with the cam surface so that the associated piston does reciprocate within its cylinder, and accordingly the cylinder is activated. When the lever is in this other operative position, the lever is elastically deformed and the lever acts as a bar spring to urge the drive means into contact with the cam surface on the member drivingly connected to the output shaft. The cylinder is provided with an auxiliary inlet valve for introducing compressed air into the cylinder above the piston to ensure that the drive rollers are maintained in continuous contact with the cam surface during certain periods of operation of the invention.
As the piston reciprocates within the bore of an activated cylinder, the piston power is transmitted through the connecting rod to the free end of the lever and the drive means or rollers carried thereby. The power is thence transmitted by the drive means to the cam surface on the member drivingly connected to the output shaft and to the output shaft itself. The cylinder has a centerline, and the components are so constructed and arranged that the lower end of the connecting rod deviates very little from the centerline of the cylinder during reciprocation of the piston within the cylinder. Side forces generated between the drive means and the cam surface are transmitted through the lever to the engine body instead of through the piston and cylinder wall as occurs with convention powertrains.
The invention advanced piston-cam assembly powertrain can achieve a combustion process under constant pressure by coordinating the rate of fuel injection with a desired piston movement produced by choosing an appropriate cam profile. Then, instead of reducing NOx formation by recycling exhaust gas (EGR), the invention offers a superior means for lowering maximum firing temperature by choosing an appropriate compression ratio such that at the end of a constant pressure combustion process, the firing temperature is within the allowable limit. Lost cycle efficiency, due to lowered firing temperature can be compensated for by an overexpanded cycle when the invention powertrain is used.
The above discussion relates to a single cylinder along with its associated components. However, in most practical applications, a plurality of cylinders are employed, a six cylinder being described in detail hereinafter. When multiple cylinders are utilized, each cylinder is individually mounted on the engine body or frame. The rotating part of the powertrain including the member and the drivingly connected output shaft have sufficient mass and angular momentum to function as a flywheel. The output shaft is supported on spaced ball bearings, and needle bearings can be used to reduce friction at each end of the connecting rod. The only sliding motion is between the short piston skirt and the cylinder wall where normal forces are very small. Therefore, engine friction losses are small and mechanical efficiency is very high. A light reciprocating mass and small piston frictional resistance allow a much higher piston speed to further boost engine specific output. Since the invention powertrain eliminates an expensive crankshaft, a separate camshaft and a gear train, the manufacturing cost of a new engine is greatly lowered.
In urban areas where the speed limit is well below 70 mph, only one cylinder of a multi-cylinder engine is sufficient to meet the power requirement. The one cylinder operation providing one power stroke per engine revolution is equivalent to a conventional 2-cylinder engine thereby providing a most effective way to minimize air pollution in the city. An electronic device based on information from a torque sensor on the power output shaft can be used to determine which cylinder or cylinders should be activated to meet the power requirements as well as to distribute the operation equally among all of the cylinders. On average, only one-third of all cylinders in a 6-cylinder engine will be activated, and thus the engine can last three times longer than a conventional engine design. Eventually, the number of automotive engine manufactures and related factories to keep the same number of automobiles on the road could be greatly reduced. A new engine, which can last three times longer, can be obtained by combining the advanced powertrain of the invention with the conventional piston-cylinder assembly of a CI engine.
The biggest advantage of a piston-cam assembly powertrain over conventional piston-crank assembly powertrains is the fact that piston speed is an independent variable. As the shaft rotates, piston speed is not determined by engine rpm but by a cam profile which can be varied as desired to make the piston speed an independent variable. For a given maximum cycle pressure, a constant-pressure cycle CI engine has higher efficiency, a greater specific power, and a lower firing temperature than either a constant-volume or limited-pressure cycle engine. Accordingly, for improved engine performance a 4SDI engine should be designed to operate on a constant-pressure cycle. At the same time, in order to reduce in-cylinder NOx formation, a 4SDI engine must also operate under a limited temperature cycle. The key to achieving both goals simultaneously is to develop an appropriate cam profile to achieve a composite combustion process with a constant pressure portion to achieve high engine performance and a limited temperature portion to reduce the in-cylinder NOx emission to a predetermined level. For diesel engines utilizing a piston-crank assembly powertrain, 95% of heat release is accomplished within about 35 degrees of crank angle. In a piston-cam assembly powertrain, a larger cam profile section can be arbitrarily allocated to fuel injection/combustion to prolong the combustion process. As a result, better fuel-air mixing and more complete combustion can be achieved to reduce levels of particulates, CO, and unburned hydrocarbons. The longer duration for fuel injection/combustion has the effect of reducing the effective expansion ratio. To boost engine performance, a larger expansion piston stroke can be chosen to operate the engine with an over-expanded cycle. The powertrain of the invention can accomplish the required characteristics of a combustion process to improve engine performance and to lower in-cylinder emission levels.